The main purpose of an exploration (e.g. offshore oil exploration) over a survey is to cover its underlying area.
As shown in FIG. 1, to ensure this goal is achieved, the area is divided into small areas called bins 2. These bins are virtually located at the bottom of the sea, and the purpose is to hit them through various kinds of reflected waves 8. These waves are captured by receivers (hydrophones) 3 that are part of different source-receiver offset classes, also referred to as “offset classes” (see vertical lines referenced 4, symbolizing the separators between offset classes). A source-receiver offset is an offset distance between source and receiver. The offset classes can possibly be one of the following: “near offsets”, “near-mid offsets”, “mid-far offsets”, “mid offsets”, “far offsets”, . . . , depending on the receiver location along the streamer (linear antennas) 7 towed the seismic vessel 6. Each bin 2 shall be hit enough through these offset classes, according to the client specification, so that the coverage can be considered correct. In fact, the wave 8 starts from the source 5, and ends at a receiver 3 after bouncing on a reflexion point 1, at the bottom of the sea (more precisely, the wave is reflected by the subsurface's layers, at a reflexion point 1). Thus, a bin 2 is considered hit when the reflexion point 1 falls into it.
Operationally, as shown in FIG. 2, the survey is discretized into preplot lines 24, 25, 26, etc. A preplot line is an arbitrary geometric curve, and can be therefore a straight line, a broken line, a circle, an ellipsoid, or something else that can be mathematically expressed through an equation of the type f(x,y)=0. Those preplot lines are positioned on the survey area in such a way that following all of them with a null steering offset (see definition below) and the streamers 7 perfectly aligned behind the vessel 6, would result in a perfect global coverage (a part of which corresponds to each of the preplot lines). In FIG. 2, the coverage corresponding to each of the preplot lines referenced 24, 25 and 26 are referenced 21, 22 and 23 respectively. Being able to steer with a null steering offset and getting streamers perfectly aligned behind the vessel is in practice impossible, mainly because of currents. In practice, preplot lines are contiguously shot so that one can adjust the vessel position to juxtapose the coverage of a given preplot line with the coverage of the adjacent preplot line(s).
In operation, the line along which the vessel actually sails is called “sail line”. The sail line is generally the preplot line, but can be different in some rare cases, e.g. if the vessel must avoid an obstacle located on the preplot line (oil platform, FPSO unit (“floating production, storage and offloading unit”), etc).
In practice, the navigation system (also referred to as INS, for “Integrated Navigation System”), which is onboard the vessel, typically receives two control settings:                the steering offset (also referred to as “desired track offset” or DTO), which is the desired offset between the sail line and the vessel; and        a distance DC, which is the current distance from the sail line to the vessel position.        
Within the navigation system, these control settings are usually used by an autopilot system (e.g. Robtrack) which determines how to alter the seismic vessel course so that the vessel reaches a new position in which the steering offset (DTO) is respected.
In a first known solution, this operation of juxtaposition is manually done aboard, by using the information given by a binning software. For the navigator (human operator), the usual process is to watch the binning software screen, spot holes (e.g. 27) in the coverage by scanning the colours, possibly anticipatively, and to adjust the vessel course accordingly. In practice, the steering offset (DTO) is constantly adjusted (visually) by the human operator.
A main drawback of the first known solution is that steering the vessel to juxtapose the coverage of a preplot line with the coverage of the adjacent preplot line(s) implies that navigators (human operators) must constantly adjust their steering offset by taking the information of the binning software, from the binning offset classes considered. Doing so manually is suboptimal and is very difficult to achieve even for an experienced user.
Another drawback of the first known solution is an excessive steering caused by the motivation of juxtaposing the coverage. Overdoing it often results in a too dynamic line, difficult to mimic in case of a future 4D survey based on the current one.
A second known solution (more recent practice) is to maintain a null steering offset with regards to the preplot line.
A main drawback of the second known solution is that setting a null steering offset indeed facilitates the 4D survey to come, but will result in a poor global coverage.
For information, a 4D acquisition well known in the field of seismic is to repeat later a 2D or 3D acquisition, in order to analyze the changes between two periods, typically to monitor a reservoir.